In the processing of melts of inorganic materials, especially of metal and glass melts, it is customary to provide the surfaces of tools, aids and especially of molds which come directly into contact with the liquid materials with a release agent. Such a release agent (which is usually referred to as a mold release agent) has the task of preventing the reaction of a melt with the surfaces mentioned. The adhesion of the melt on these surfaces should be very low both in the liquid state and in the solid (cooled) state. For example, a casting can be removed more easily from a mold provided with a mold release layer. A mold release layer reduces the wear on a mold and correspondingly also has a positive effect on its lifetime.
Mold release layers should not adhere on the surfaces of castings and, if they are applied as antiwear sizes, also should not become too firmly bonded to the surfaces of the molds, tools or aids. In addition, a mold release layer should be noncombustible and additionally environmentally compatible, which means especially that no toxic substances should outgas at high temperatures. In general, mold release layers are produced in very homogeneous layer thicknesses by painting on or spraying on a paintable or sprayable composition. Such a composition for producing mold release layers is also referred to as a size.
The prior art discloses both purely organic and inorganic mold release layers. U.S. Pat. No. 5,076,339 describes, for example, an organic mold release layer based on low-melting waxes. Such organic release layers are, however, immediately decomposed thermally on contact with metal or glass melts at a temperature of several hundred degrees. Between a mold provided with the organic release layer and the melt, a gas cushion may form, which can in turn lead to the formation of pores in the casting.
Inorganic mold release layers are generally preferred for this reason in the processing of hot metal or glass melt.
Commercially available inorganic mold release layers are based usually on the following compounds: graphite (C), molybdenum disulfide (MoS2) and boron nitride (BN), the latter especially in its hexagonal form. Mold release layers based on these materials are notable for their exceptionally low wettability by metal melts. They have particularly low wettability with respect to aluminum and magnesium melts and with respect to melts composed of aluminum-magnesium alloys. While, however, graphite is oxidized under air even at temperatures around 500° C., and molybdenum sulfide even at as low as from 400° C., boron nitride is stable under the same conditions up to about 900° C. Accordingly, boron nitride is suitable especially as a constituent of mold release layers for high-temperature use.
However, both mold release layers based on graphite or molybdenum sulfide and those based on boron nitride are generally not very abrasion-resistant. Especially the processing of light metal melts or glass melts (melts in which high flow rates occur) places high mechanical demands which are not met in a lasting manner by the known inorganic mold release layers. The mold release layers known from the prior art are correspondingly worn very rapidly, are generally not intended for repeated use for this reason and have to be replaced regularly.
It could therefore be advantageous to provide a mold release layer which does not have the disadvantages known from the prior art. A mold release layer should be very inert and oxidation-resistant with respect to metal melts. In particular, a mold release layer should have low wettability with respect to metal and glass melts and simultaneously be abrasion-resistant. It should be able to withstand high mechanical stresses which arise, for example, in the course of processing of melts with high flow rates, and are thus suitable for repeated use (even in the case of lasting stress over several days and weeks).